Journal of Hydraulic Engineering

This biography of Garbis H. Keulegan (1890‐1989), a leading figure in hydraulics and fluid mechanics for more than half a century, recounts his early years in Armenia; immigration to the United States; education; military service; employment and technical activities at Westinghouse, the National Bureau of Standards (NBS), and the Waterways Experiment Station; awards and recognition; and retirement at age 98 after becoming the oldest and longest‐serving (67 years) employee in federal service. His graduate study, work at Westinghouse, and early research at NBS were on solid mechanics. Following his assignment as one of the first staff members of the NBS National Hydraulic Laboratory, his progressive shift of interest to fluid mechanics became complete. At NHL he did his classical work on turbulent open‐channel flow, water waves and tides, flow in curved pipes, and salinity intrusion. During World War II he was seconded to the Beach Erosion Board to provide technical guidance for amphibious military landings. Following his mandatory retirement from NBS, he was engaged by WES to continue his work on waves and tides, density currents, and salinity intrusion, and produced his classical analysis of flow through tidal inlets.

In this paper, results on the motion of two‐dimensional dense thermals descending on slopes of less than 5° are presented. The front propagation, concentration distribution, and internal structure of the thermals are investigated experimentally and analytically. Laboratory experiments are carried out in a 20‐m‐long water tank to determine the front velocity and the temperature field in cold, saline thermals. The similarity assumption is employed in the analysis of the results. The front velocities of the thermals agree well with analytical results, and the influence of the water depth at the source on the rate of advance of the front is insignificant. The similarity concept is shown to result in contradictions when applied to depth‐averaged parameters in the tail of the thermals. The internal structure of thermals is further examined by using laser‐induced fluorescence (LIP) flow visualization in a 1‐m‐long aquarium. The photographs show that three‐dimensional structures form at the front and engulf ambient fluid. The frontal part has oscillatory propagation of lobes and the ensuing development of large billows.

The motion of an air cavity, formed when a gate is opened at the lower end of a nearly horizontal, water‐filled square duct, is compared to a gravity current, formed by the interaction of two liquids of differing density. If the gate is opened to the full depth of the duct, the front of the cavity has the same shape and speed as a gravity current. Behind the front, a quasi‐steady, gradually varied flow extends to the outlet. If the gate is only partially opened, it forms a weir at the lower end that controls the depth of the outflow. A low weir produces a jet deflection and no effect on the cavity shape. As the height is increased, a level is reached where a hydraulic jump is formed. If the weir height exceeds a value dependent on the duct slope, the free surface touches the roof and seals the cavity. The isolated air cavity then progresses upstream and a new cavity forms at the opening. Simple analyses can predict the shape and propagation speed of a cavity created by free overfall and some of the physical characteristics of a cavity created when a weir controls the outflow depth.

A review of Keulegan's contributions and influence in the field of stratified flow is presented. Emphasis is placed on the mechanics of the saline wedge, a topic in which he was actively engaged in research for 40 years. Keulegan's early analysis and experimental findings on the saline wedge are contrasted with recent theory, experimental studies, and field applications. The major difference in the early and later studies is in defining the role of the Reynolds number in the linking of laboratory and field work. The paper concludes with an account of the phenomenon of “dead water” encountered by a ship traversing the two‐layer, salt‐fresh‐water stratification in the mouth of the Orinoco River in Venezuela. In understanding this phenomenon, use is made of some stratified flow experiments performed by Ekman at the turn of the century.

The significance of the Keulegan‐Carpenter parameter $(K=U_{m}T/D)$ to computing wave loads are reviewed. When the momentum transport theorem is scaled, K is the ratio of the flux of momentum to the local acceleration in the total inertia of the fluid; or equivalently, the ratio of the drag force to the inertia force in the Morison equation. When the condition of the data is measured by the Dean eccentricity parameter $(E=\sqrt{3}K/2{\pi }^2)\text{,}$ $K=11.40$ determines when the data are equally conditioned for computing $C_m$ and $C_d\text{.}$ This is approximately the same value of K that Keulegan and Carpenter identified as a “critical” value where the maximum $C_d$ and the minimum $C_m$ occurred in their data for horizontal cylinders. When stable transverse lift forces are due to the shedding of a single eddy during one‐half of a wave period, $K\approx 11$ when the Strouhal parameter is $0.18<S<0.20\text{.}$ The elevation of values of $C_d$ in the supercritical region for $R>{10}^6$ is due to: (1) Roughness effects; and (2) wake effects that correlate with K. The need to explore the critical values between $11.40<K<13.16$ when the Reynolds parameter $R>{10}^6$ is demonstrated by the design specifications for a typical jacket‐type offshore structure.

Wind‐driven currents in estuaries, lakes, and the ocean are often observed to include strong longitudinal vortices, known as Langmuir cells. To test experimentally the theory developed by Craik and Leibovich that these cells arise through wave‐current interactions, we superimposed mechanically generated waves on a turbulent channel flow. Our results show that streamwise vortices can form through wave‐current interaction described by Craik and Leibovich: The vertical component of the mean vorticity can be rotated by the sheared Stokes' drift current of the waves to produced longitudinal vorticity. In the present configuration, the main sources of vertical vorticity are the boundary layers on the sides of the channel. Nonetheless, given the fact that longitudinal vortices like these enhance mixing, our experiments suggest that wave‐current interactions can contribute significantly to near‐surface mixing in many water bodies. Thus, the connection between waves and mixing that Keulegan and Brame sought to make when they studied mixing by wind waves does seem to exist, albeit with a somewhat different physical basis than they described.